Ministry of Environment Terrain

Character of Surficial Materials

The chief criterion used for subdividing the land surface into terrain units is surficial material characteristics. Surficial materials are classified according to their mode of origin (genesis) because there is a close relationship between the process whereby they were formed and their most important physical properties.

MORAINAL MATERIALS (TILL) (M) were deposited either beneath (basal till), upon (ablation till), or at the snout (end moraines: Mr) of a glacier.

Basal till is widespread in British Columbia. It occurs as a mantle overlying bedrock (Mb, Mv), and as till plains (Mp, Ml), drumlins (Mr, p ) and fluted moraine (Mm, f ). It typically consists of rock fragments of many sizes and shape in a matrix of sand, silt and clay. The proportions of these constituents and related till properties vary in accordance with the geology of the source area. Granitic rocks give rise to sandy, bouldery tills; limestone, many volcanic rocks and shales produce tills with a silty or clayey silt matrix. In many valleys, river sands and gravels and lacustrine silts were reworked by glaciers to form a till that consists of rounded stones in a sandy silt matrix.

Basal till is typically the most cohesive and most highly consolidated of all surficial materials. It has low permeability and may be effectively impermeable if clay content is high. Depressions, lower valleysides and level areas on basal till may be poorly drained, and the water table may be close to the ground surface during rainfall or snowmelt. The bearing strength and shear strength of basal till are relatively high, and consequently this material is good for foundations and road beds, provided that sites are well drained. Basal till is also suitable for fill, although unvegetated embankment slopes are prone to erosion or to failure when saturated. Difficulties may arise when excavating till with earth moving equipment due to its hardness or the presence of large boulders.

Ablation till is comparatively loose, non-compact, and low in silt and clay. Its properties are more similar to those of glaciofluvial gravel than to those of basal till.

FLUVIAL SEDIMENTS (ALLUVIUM) (F) result from deposition bv streams and rivers. They comprise floodplains and alluvial plains (F^Gl, Fl), river terraces (Ft), alluvial fans (Ff, F^Gf), and deltas (Ff, F^Gf, Fl, F^Gl, Fp). They consist of rounded gravel, sand, and silt.

Fluvial deposits are non-cohesive and have low shear strength. Gravels and sands are highly porous and permeable, and permit rapid drainage and groundwater flow. Their bearing strength is generally high. Fluvial sediments are easily excavated by earth moving equipment, although vertical cuts in sand and gravel are modified by dry ravel until a slope of about 35° is attained. Locally, coarse gravels (bF, bgF) may pose problems for excavation.

River terraces generally provide dry, stable sites for construction. Appropriate set-backs should be made at the top of terrace scarps that are being undercut by a river.

Floodplains and "active" fluvial fans are affected by floods and shifting channels. These areas also tend to be poorly drained since the water table lies close to the ground surface. Detailed studies of flood hazard and site conditions must be carried out prior to development on this kind of terrain. Detailed floodplain maps for some major rivers in British Columbia are available from the Water Management Branch, Ministry of Environment, Victoria, British Columbia, V8V 1X5.

GLACIOFLUVIAL SEDIMENTS (F^G) were deposited by meltwater streams in close

proximity to glaciers. Their composition, properties and land use potential are similar to those of fluvial materials. Irregular topography (F^G h, F^Gr, F^Gm) may limit development or require much excavation for site preparation. Glaciofluvial terraces and outwash plains (F^G t, F^Gl, F^Gp) resemble river terraces and fluvial plains, but they include depressions which may be poorly drained or contain lakes. Groundwater may be present at depth in these gravels and sands.

COLLUVIUM (C) results from accumulation of material that moves downslope due to gravity. There are three groups of colluvial deposits.

The first group consists of talus slopes (Ca, Cc), colluvial mantles (Cv, Cb, Cw), snow avalanche deposits (e.g. Cf-A), and rockslide debris (e.g. aCh). These typically consist of angular rock fragments with interstitial sand and silt. The material is loosely packed, non-cohesive, highly porous and permeable, and well-drained; bearing strength is moderate to 6 Character of Surficial Materials high. Some forms of construction, such as roads, are feasible. Development on these types of terrain, however, is severely limited by steep slopes and potential hazards which include rockfalls, rockslides and snow avalanches. Site specific investigations are necessary before development.

The second group of colluvial deposits consists of debris flow or mudflow fans (Cf, dCf, rsCf). These are steeper (8° to 14°) than most alluvial (fluvial) fans, but less steep than talus slopes and avalanche cones. They generally consist of interlayered debris flow deposits and thinner fluvial gravels; the former are relatively cohesive and their bearing strength is relatively high. Colluvial fans are well drained. They provide substrate suitable for development, but this may be offset by the threat of further debris flows. An examination of conditions in basins above fans is necessary in order to assess this hazard.

Earthflows and slumps constitute the third group of colluvial deposits (e.g., Ch, Cm, Cr). The texture and physical properties of these materials depend upon the geology of their source areas. They commonly consist of weathered bedrock and/or till with a high clay content. Many earthflows and slumps are undergoing slow movement at the present time (e.g., dCmr-F). Special investigations are needed before development occurs on this type of colluvium.

LACUSTRINE AND GLACIOLACUSTRINE SEDIMENTS (L, L^G) accumulated as a result of settling of fine particles from suspension in lakes, chiefly ice- dammed lakes during deglaciation. The sediments consist of thinly interlayered fine sand, silt and clay. They are weakly consolidated with relatively low bearing strength. Silts and clays are cohesive and of low permeability so that level areas and depressions are often wet and poorly drained.

Slopes and terraces underlain by lacustrine sediments are readily affected by surface erosion which results in the formation of deep gullies (Lt-V). Steep slopes such as terrace scarps are also subject to slides and slumps, particularly where groundwater seepage occurs near the foot of the scarp. Piping (underground erosion resulting in surface collapse) takes place within lacustrine terraces (Lt-P). Local dust hazard may exist where vegetation is sparse.

Lacustrine materials are generally unsuited for those kinds of development that aggravate their natural tendency for instability and erosion. Addition of load to slopes and additions to groundwater from irrigation, storm sewers and septic tanks should be carefully planned.

EOLIAN DEPOSITS (E) consist of coarse silt and sand transported and deposited by wind. They commonly occur as mantles on river terraces and inactive fans (e.g., sEv) where they constitute parent material for loamy, stone-free soils. Sand dunes (e.g., sEm) are found in windy locations where there is (or was) a suitable source of sediment.

Eolian sands are non-cohesive and subject to surface sliding, although cementation by calcium carbonate may stabilize low cut-banks in some areas. Eolian sediments in general are non-consolidated and subject to compaction when loaded. They are normally so thin, however, that foundations rest in the underlying material. Where vegetation is sparse or absent, dust may be a problem.

VOLCANIC MATERIALS (V) consist of unconsolidated pyroclastic deposits (ash and cinders), and recent lava flows with a surface cover of loose, blocky debris.

ANTHROPOGENIC MATERIALS (A) consist of natural materials that have been so disturbed, modified or relocated that their physical properties have been significantly changed. This category includes spoil heaps, settling ponds, open pit mines, and areas of landfill. It does not include urban areas where buildings are constructed upon the natural substrate.

MARINE AND GLACIOMARINE SEDIMENTS (W, W^G) result from settling of sediment from suspension and from the melting of floating ice during deglaciation, and wave action. The former processes result in deposition of silts and clays and stony silts and clays. These have properties that are similar to those of lacustrine sediments, including a susceptibility to erosion, instability, and a tendency to subside when loaded. They are relatively impermeable and poorly drained.

Wave action has formed beaches, bars, and spits of gravel and sand along both the present shoreline (e.g., gW^Gm) and older, slightly higher shorelines (e.g. gWm) where they constitute relatively dry and well drained terrain.

BEDROCK (R) is mapped where rock outcrops at the ground surface or is covered by less than 10 cm of surficial material. Rock characteristics such as hardness, strength, drainage and rippability vary in accord with rock type. Bedrock geology maps should be consulted where rock outcrops are extensive.

WEATHERED BEDROCK (D) is mapped where mechanically and chemically weathered debris is derived in situ from underlying bedrock. Generally applied to flat or gently sloping terrain because weathered bedrock that has moved downslope is classified as colluvium.

ORGANIC MATERIALS (0) result from the decay and accumulation of vegetation in closed basins or on gentle slopes. Such areas tend to be poorly drained, or even inundated for parts of the year. Bearing strength of organic sediments is very low, and much compaction occurs when they are loaded. When organic sediments are artificially drained or dried, shrinkage and subsidence occur.

Organic terrain does not provide suitable sites for any kind of development unless considerable modification (such as drainage and compaction) is carried out before construction.

ICE (I) is indicated on terrain maps where glaciers and snowfields occur.